Asia’s Scientific Trailblazers: Xue Qi-Kun

Professor Xue Qi-Kun shares the story behind his discovery of quantum anomalous Hall effect and other developments in the field of condensed matter physics.

Xue Qi-Kun
State Key Lab of Quantum Physics
Tsinghua University

AsianScientist (Jun. 7, 2017) – Professor Xue Qi-Kun may have already succeeded in performing one of the most challenging experiments in condensed matter physics but he is by no means resting on his laurels. One of two winners of the inaugural Future Science Prize, Xue is recognized for his trailblazing work on high-temperature superconductors.

In this interview for Asia’s Scientific Trailblazers, Xue tells Asian Scientist Magazine more about the heady excitement of working on superconductivity in the golden age of the discipline, and how that enthusiasm has continued on to his present research.

  1. How did you become interested in condensed matter physics?
  2. In 1987, I started my research as a PhD candidate at Institute of Physics. As you know, that was the golden age for high-Tc superconductivity; exciting results kept coming out and everyone got so excited at this field of condensed matter physics. More that just because it was the ‘Woodstock of physics’, I was captivated by the possibility that our discoveries could change human life!

  3. What is molecular beam epitaxy and how have you used it in your work?
  4. Molecular beam epitaxy (MBE) is a state-of-the-art technique, which allows us to grow high quality single-crystalline films on certain crystal substrate using molecular beams. One of the great advantages of MBE is that it can control film thickness down to single atomic layer. It is like painting or printing, but your ink is a single layer of atoms, with which you can easily build or even design your materials precisely. That’s exactly what we have achieved in our work on quantum materials such as high-Tc superconductors and topological insulators.

  5. What is the quantum anomalous Hall effect and why is it important?
  6. By definition, the quantum anomalous Hall effect (QAHE) is one kind of quantum Hall effect (QHE) that occurs with no external magnetic filed. Carriers with locked spin and momentum in quantum Hall state can only travel along the physical edges of the system, just like the vehicles on the highway. In other words, carriers in a quantum Hall state behave well and dissipate much less energy compared with those in normal transistors, which make them ideal for applications in low-power-consumption electronic devices. However, to realize the QHE, very high magnetic fields of about 10 Tesla is needed. This is one of the reasons why QAHE is so important.

    Prof. Xue Qi-Kun inside his laboratory at Tsinghua University.
  7. What was so significant about your discovery of the quantum anomalous Hall effect in 2013?
  8. We realized QAHE in a magnetically doped topological insulator (Bi,Sb)2Te3 system. It is a two dimensional system with the thickness of only five nanometers. Magnetic elements were introduced to form the long-range ferromagnetism, which then caused the spin-polarized carriers.

    The most difficult part of experiment was growing ferromagnetic but bulk-insulating material. Another challenge was to determine the ‘correct’ composition of four elements: bismuth (Bi), antimony (Sb), tellurium (Te) and chromium (Cr). All these made the experiment one of the most challenging experiments in condensed matter physics in last ten years or so. Eventually, we succeeded.

  9. Tell us more about your research on high temperature superconductivity.
  10. My group has been working on superconductivity for ten years and our ultimate goal is to reveal the mystery of high Tc superconductivity and discover new superconducting materials with transition temperature Tc (much) higher than 77 K (the liquid nitrogen boiling temperature). Science is always beautiful but simple. I believe that phonons are the only important media to glue the electrons together in superconductors, as indicated by Bardeen–Cooper–Schrieffer (BCS) theory. We are working very hard to prove this idea.

    I would like to share with you two systems related to this. The first one is superconductivity enhancement at the interface between one layer FeSe and SrTiO3. By using MBE, we deposited 0.55 nm thick FeSe film on STO, and found that its superconductivity transition temperature is higher than 65 K while the Tc of bulk FeSe is only 8 K. It is already the highest record for iron-based superconductors. And possibly, it could be the second system like cupartes with the Tc above the boiling temperature of liquid nitrogen.

    The second one is about cuprates. I have a very simple model to understand the high-Tc problems. We have observed evidences for the s-wave pairing in cuprates. For decades, I think many studies on cuprates have been based on the information of BiO layers. BiO layer is the most easily obtained surface after mechanical cleavage of BSCCO.

    However, all the physics of superconductivity happens in its underlying CuO2 layers, which, unfortunately, has not been fully understood yet! By using MBE, we have already obtained the CuO2 layer. By doing so, we are getting more and more evidences for s-wave pairing in the system. All our findings support my model. I have confidence on that!

  11. What developments in the field of 2D materials would you like to see in the next decade?
  12. In addition to revealing the mystery of high-Tc superconductivity, I hope that we will be able to detect Majorana fermions and perhaps even realize quantum computing.

  13. What impact has winning the Future Science Award had on you personally and on your career?
  14. I really appreciate the founders and the donators of the Future Science Award. As a scientist, it is certainly greatest honor and encouragement. More importantly, the Future Science Award has opened a new charter in the history of Chinese science. The whole society may respect science much more, and attract more young people dedicated to science. Regarding to the fast-developing China economy, it was just in time!

  15. If you were a young graduate student today, what problem would you choose to work on and why?
  16. It is difficult to say, but I would probably still be a scientist working on superconductivity at room temperature.

    This article is from a monthly series called Asia’s Scientific Trailblazers. Click here to read other articles in the series.


    Copyright: Asian Scientist Magazine; Photo: Xue Qi-Kun.
    Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

Rebecca did her PhD at the National University of Singapore where she studied how macrophages integrate multiple signals from the toll-like receptor system. She was formerly the editor-in-chief of Asian Scientist Magazine.

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